1. Field of the Invention
The present invention relates to a semiconductor light emitting device employing III-V nitride compound semiconductor.
2. Description of the Background Art
In recent years, active research and development has been being conducted in semiconductor lasers employing III-V nitride compound semiconductor such as AlGaInN as semiconductor lasers capable of emitting light in an area ranging from the blue region to the ultraviolet region necessary for achieving higher density of optical disks, and such semiconductor lasers have already been put into practical use.
Semiconductor lasers employing III-V nitride compound semiconductor which have been reported so far have a separate confinement heterostructure (SCH).
More specifically, to effectively confine light in an active layer, an n-type AlGaN cladding layer made of a material having a relatively low refractive index is provided on the n-side, while a p-type AlGaN cladding layer made of a material also having a relatively low refractive index is provided on the p-side.
Further, an n-side optical guiding layer made of a material having a relatively high refractive index is provided between the n-type AlGaN cladding layer and active layer, while a p-side optical guiding layer made of a material having a relatively high refractive index is provided between the p-type AlGaN cladding layer and active layer.
An AlGaN cladding layer has a lower refractive index with increasing Al composition ratio. Accordingly, the use of an AlGaN cladding layer having a high Al composition ratio allows light distribution to converge to the vicinity of the active layer. This is because light attenuation with distance from the active layer increases with growing difference between an effective refractive index in optical mode and refractive index of a material. This increases optical confinement in the active layer, which advantageously reduces a threshold value.
On the n-side, a GaN material is generally stacked on a surface of the n-type AlGaN cladding layer farther away from the active layer. In the case of employing a substrate such as sapphire or SiC which produces a great lattice mismatch with GaN, a low-temperature GaN buffer layer for lessening the lattice mismatch is stacked between the substrate and cladding layer. In the case of employing a dislocation-reducing technique or the like using a lateral growing method of GaN material, a GaN lateral-grown layer having a thickness of several micrometers or more is stacked between the substrate and AlGaN cladding layer. In the case of employing a GaN substrate, which has become used popularly in recent years, the GaN substrate shall also be present under the n-type AlGaN cladding layer.
As described above, in the case where a GaN material or a material having a refractive index higher than the effective refractive index in optical mode is provided on the surface of the n-type. AlGaN cladding layer farther away from the active layer, such material shall have a high light-confinement coefficient since light intensity is less likely to be attenuated even with distance from the active layer. A problem is known in that light confinement in the active layer thus decreases relatively, causing characteristic degradation such as a significant increase in threshold value (cf. Japanese Journal of Applied Physics Vol. 38 Part 1, No. 3B (1999) p. 1780-).
Since Fresnel reflection of light occurs at the interface between the GaN layer and a sapphire or SiC substrate having a different refractive index from the GaN layer or occurs on the lower surface of the GaN substrate, a resonant mode is created within the GaN layer or GaN substrate. The resonant mode has a problem of causing a ripple in a far-field pattern (FFP) in the vertical direction, which has been confirmed by actual measurement, simulation and the like.
To solve such problems, it is necessary to minimize the spreading of light into a GaN material provided on the surface of the n-type AlGaN cladding layer farther away from the active layer or a material having a refractive index higher than the effective refractive index of an optical mode. Therefore, the n-type AlGaN cladding layer needs to be increased in Al composition ratio, that is, needs to be reduced in refractive index to significantly attenuate the light intensity with distance from the active layer so that the light intensity is sufficiently attenuated within the n-type AlGaN cladding layer. To significantly attenuate the light intensity with distance from the active layer, it is preferable to form the n-type AlGaN cladding layer as thick as possible.
On the other hand, the GaN buffer layer grown at low temperatures on a sapphire or SiC substrate and the GaN layer or GaN substrate grown on the sapphire substrate by the lateral-growing method have a lattice constant very close to that of GaN.
When growing the n-type AlGaN layer on these layers as an n-type cladding layer, the lattice constant of the AlGaN material decreases with increasing Al composition ratio. Thus, the lattice mismatch with the underlying layer increases with increasing Al composition ratio. As a result, it is known that the occurrence of cracks or dislocation become significant. Even when neither cracks or dislocation occurs, a great level of distortion occurs, which adversely affects the life of elements.
As described, in the case where the Al composition ratio of the n-type AlGaN cladding layer is excessively increased, a film thickness (critical film thickness) that can be grown without causing cracks or dislocation decreases. Thus, on the contrary, light is more likely to spread out into the substrate.
In light of the foregoing, it is generally known that the n- and p-type AlGaN cladding layers each have an Al composition ratio of optimum value, and an AlGaN material having an Al composition ratio of approximately 0.06 to 0.07 is used for both the n- and p-type AlGaN cladding layers (cf. T. Tojyo, et al. “High-Power AlGaInN Laser Diodes with High Kink Level and Low Relative Intensity Noise”, Jpn. J. Appl. Phys. Vol. 41 (2002), pp. 1829-1833).
However, FFP in the vertical direction needs to be taken into account in determining the Al composition ratio of the n- and p-type AlGaN cladding layers. Generally, in nitride-based laser diodes for application in optical disks, the angle at full width at half maximum of FFP in a direction parallel to the substrate ranges approximately from 6 to 10° while the angle at the full width at half maximum of FFP in a direction perpendicular to the substrate is 20° or more. In this manner, beam-outgoing angles in the horizontal and vertical directions are significantly different from each other.
For application in optical disks, however, the ratio of angles at the full width at half maximum of FFP in the vertical and horizontal directions (aspect ratio) is required to be as close as possible to one. Therefore, it is preferable to minimize the full width at half maximum of FFP in the vertical direction.
Generally, a light distribution within a semiconductor laser element, that is, a near-field pattern (NFP) and FFP are in the relation of Fourier Transform. Therefore, to minimize the full width at half maximum of FFP in the vertical direction, NFP needs to spread out widely. This is achieved by, for example, a method of reducing the refractive index of an active layer to cause light to spread out widely in the vertical direction. This case, however, inevitably causes a problem of increased light absorption resulting from the wide spreading of light into a p-type contact layer or p-type electrode as well as the aforementioned problem of spreading of light into the substrate.
To solve these problems, the n- and p-type AlGaN cladding layers need to be increased in Al composition ratio or need to be formed thick, which, however, disadvantageously causes cracks or dislocation.
The above-described problems result from a specific structure of a semiconductor laser or semiconductor light emitting diode employing III-V nitride compound semiconductor in which a layer (e.g., GaN buffer layer or GaN substrate itself) having a refractive index higher than the effective refractive index is present closer to the substrate than the n-type cladding layer, and in which AlGaN cladding layers have a lattice constant different from that of underlying GaN.
As described, the spreading of light into the substrate, the occurrence of cracks or dislocation resulting from the lattice mismatch with the underlying layer and the problem about the full width at half maximum of FFP in the vertical direction are inseparably related to one another. To solve all these problems, a specific technique is required that can control the occurrence of cracks or dislocation even when the AlGaN cladding layers have a high Al composition ratio.